I've heard of it referred to as a check digit, or in the old telecom days, as the CRC - Cyclic Redundancy Check. Interesting that there are so many names for essentially the same thing - a crude validation of the integrity of the value string that preceded it.
I don't have easy access to documentation right now (brand new computer!!) but it should be possible to write a function to return the binary sum of all prior bytes as a four-hexadecimal string. Checking Harbour's HexaToDec() or DecToHexa() might help, or at least give inspiration that would lead to a solution?
Hope that helps!

The description of BCC in the manual (translated from original Spanish) is as follows:

BCC = Four hexadecimales. Binary sum of all prior bytes

In my opinion the binary sum of all bytes is nothing but the sum of the ASCII codes of each character in the string.
From a mathematical point of view, it does not matter if we add up the number of binary, hexadecimal or decimal notation. The sum should always be the same.
For example:
the sum in binary notation : 101 + 110 = 1011; decimal = 11
the sum in decimal notation: 5 + 6 = 11

I'm surprised the given example, because the sum of all bytes (from [STX] to [ETX]) results in a hexadecimal notation 0554, not 0054.